1. Field of the Invention
The present invention relates to a circuit board for mounting a component such as a semiconductor electronic device and, more particularly, to a circuit board for use, at high voltage.
2. Description of the Related Art
Circuit boards constructed with a metal plate such as copper bonded to a base substrate of alumina or aluminum nitride are now widely used in a power transistor module or a switching power supply module. Various structures of circuit boards of this type, and various production techniques thereof, are known in the art. Some of these are disclosed for example in: (1) Print Circuit Board Handbook, Second Edition, Completely Revised, Japan Print Circuit Industry Association, 1987; (2) Japanese Patent Publication No. 60-4154 (1985); and (3) Journal of the Institute of Metals, Vol. 22, No. 1, pp. 3-7, 1983. FIGS. 17A and 17B illustrate an example of a conventional aluminum nitride circuit board, wherein FIG. 17A is a plan view and FIG. 17B is a cross-sectional view taken along the line 17B--17B of FIG. 17A. In FIGS. 17A and 17B, a copper plate is bonded to a surface of an aluminum nitride substrate 1 serving as a ceramic substrate so as to form a collector electrode 2. The other surface opposite to the collector electrode 2 is entirely covered with another copper plate so as to form a ground electrode 3. A power semiconductor device such as an insulating gate bipolar transistor (hereafter referred to as an IGBT) 4 is attached to the collector electrode 2. On the aluminum nitride substrate 1, there is also a copper plate bonded to an area adjacent to the collector electrode 2 so as to form an emitter electrode 5. In the final product form, the circuit board is placed in a power module package in such a manner that the ground electrode 3 is connected to a block serving as a heat sink (not shown) or the like. That is, the circuit board, on which the semiconductor device 4 is mounted, is placed in a plastic package, and the inside of the package is filled with a resin such as silicone gel or epoxy resin (not shown) so that the surfaces of the circuit board are covered with the resin.
FIG. 18 illustrates an example of a production process flow of a circuit board according to a conventional technique. In this specific example, an electrode pattern is formed by means of etching. First, an aluminum nitride substrate 1 is prepared. A soldering material to be used to bond copper plates 6 and 7 to the aluminum nitride substrate 1 is printed on the surfaces of the aluminum nitride substrate 1. The thickness of the copper plates 6 and 7 is typically of the order of 0.3 mm. Copper plates 6 and 7 are placed on the aluminum nitride substrate 1 and heated at a high temperature so that these copper plates 6 and 7 are bonded to the aluminum nitride substrate 1. A resist 8 is then coated on both surfaces, and exposed to light so as to form a predetermined pattern in the resist. After developing the resist, the copper plates are etched using the resist pattern as a mask so as to remove unnecessary parts of the copper plates. The surfaces of the copper electrodes are polished, and plated with Ni. Thus, a complete circuit board is obtained.
FIG. 19 illustrates another example of a production process flow of a circuit board according to a conventional technique. In the previous example described above, the electrode patterns are obtained by etching the copper plates 6 and 7 which have been previously bonded to both surfaces of the aluminum nitride substrate 1. In the present example, unlike the previous one, copper plates are first formed into predetermined patterns and then bonded to a ceramic substrate. The patterning of copper plates into electrode patterns can be performed by means of punching or etching. After the patterning process, a soldering material for use to bond the electrode patterns to the ceramic substrate is coated on the ceramic substrate. The copper electrodes are then placed on the ceramic substrate, and heated at a high temperature so that the copper electrodes are bonded to the substrate. Thus, a complete circuit board is obtained.
When an IGBT is in an on-state, a high current of the order of 100 A flows through the IGBT. On the other hand, when the IGBT is in an off-state, a high voltage of the order of 2 kV or greater is applied to the IGBT. Thus, a high voltage appears between the collector electrode 2 and the ground electrode 3 on the circuit board. Therefor, a circuit board for mounting a power semiconductor device such as an IGBT should meet the following requirements:
1) The circuit board should have as good heat removal characteristics as possible to prevent the increase in temperature. To achieve this purpose, aluminum nitride having a high thermal conductivity is preferably employed as the substrate material.
2) The copper plates and the aluminum nitride substrate should be thin enough to prevent generation of cracks during heat cycles due to stress caused by the difference in expansion coefficients of the copper plates and the substrate.
3) The electrode patterns of the circuit board should be designed so that no discharge occurs at any portion of the electrode patterns during operation of the semiconductor device.
As a result of recent advances in semiconductor device technology, a circuit board for mounting a semiconductor device designed to operate at a voltage greater than 3 kV has been required. The increase in operating voltage of semiconductor devices has brought about problems of dielectric breakdown and partial discharging, which are not significant problems in conventional circuit boards. These problems cause a great reduction in reliability of circuit boards. The partial discharge refers to a discharge that occurs at a localized portion of a circuit board where a high electric field is concentrated. The partial discharge generates noise which can cause a semiconductor device mounted on a circuit board to operate in an erroneous manner. If partial discharge continues for a long time, degradation occurs in an insulating material, which can finally result in dielectric breakdown. If dielectric breakdown occurs in a circuit board, an apparatus or system including that circuit board can no longer operate in a correct manner. Thus, with the increase in the operating voltage of semiconductor devices, there is an increasing need for a circuit board having a high dielectric breakdown voltage and also having a high partial discharge voltage.
In conventional circuit boards, semiconductor devices mounted thereon are not operated at a very high voltage, and thus the partial discharge is not a significant problem. Therefore, the partial discharge is not taken into account in the design of the conventional circuit boards. The inventors of the present invention have investigated the effects of the partial discharge on the circuit board. First, the voltage at which a partial discharge starts was measured for conventional circuit boards whose patterns were formed by means of etching. In the case of an aluminum nitride substrate having a thickness of 1.0 mm, a partial discharge was observed to start at a voltage as low as about 5 kV. To find the cause of the low starting voltage of partial discharge, the cross-section at an end of the electrode pattern was observed. The observation has revealed that the electrode pattern 2 has a very sharp edge 2a in a portion in contact with the ceramic substrate 1 as shown in FIG. 20. In the case of a copper electrode pattern having a thickness of 0.3 mm on an aluminum nitride substrate having a thickness of 1.0 mm, the radius of curvature of the sharp edge 2a is as small as 0.01 mm. Since the electrode 2 has a certain finite thickness, the electrode 2 has another sharp edge 2b on the side opposite to the sharp edge 2a in direct contact with the substrate 1. The etching process generally occurs in an anisotropic fashion, and thus the copper pattern is etched not only in a direction across its thickness but also in a lateral direction. As a result, the etched copper pattern has a sharp edge in cross section in a portion in contact with the ceramic substrate 1. This phenomenon is well known in the art as described for example in Print Circuit Board Handbook, Second Edition, Completely Revised, Japan Print Circuit Industry Association, 1987. However, little has been known about the magnitude of the electric field at the sharp edge and its effect on the discharge. To obtain knowledge about these, the inventors of the present invention have performed numerical analysis on the electric field at the sharp edge 2a in contact with the substrate 1. The result shows that a very high electric field can occur at such a sharp edge. When a high voltage, for example 5 kV, is applied to the electrode 2, the maximum electric field near the edge 2a can be as large as 80 kV/mm. This high electric field near the edge 2a causes a reduction in the starting voltage of partial discharge. Thus, the analysis has shown that the low starting voltage of partial discharge in the conventional circuit boards is due to the concentration of the electric field at the sharp edge of the electrode pattern.
In an electrode pattern on the conventional circuit board, as described above, a very sharp edge is produced at an end portion in contact with an insulating substrate. The sharp edge of the electrode facing the ground electrode causes a concentration of lines of electric force at that edge, and thus a high electric field is produced there.
A discharge occurs in a gas space near the sharp edge of the electrode pattern. This means that if the gas space, in which a high electric field is present, is eliminated, then the circuit board will have a higher starting voltage in discharge. To eliminate the gas space, the circuit board is usually housed in a case filled with a silicone gel 9 as shown in FIG. 21. However, in some cases, a small void 10 is produced in the gel 9. The electric field in the void 10 is generally higher than that in the dielectric surrounding the void 10. Therefore, if such a void is present near the pattern electrode 2 where an electric field is concentrated to a high level, a discharge is induced more easily, which results in a reduction in the reliability. Thus, the generation of such a void 10 in a gel 9 in a high electric field region is another problem in the conventional circuit boards.